1. Field of the Invention
This invention relates to an auxiliary light emission device for focus detection suitable for providing a pattern-reflected image to a focus detecting device having a plurality of distance measuring points or a wide range of distance measuring field.
2. Related Background Art
Heretofore, it has been practised to project a pattern for the detection of the focus-adjusted state of an objective lens to an object of small contrast or an object of low luminance or for the detection of the object distance. Japanese Patent Publication No. 49-19810 and Japanese Laid-Open Patent application No. 55-111929 are known, and the assignee of the present invention proposed the technique of this kind in U.S. applications Ser. Nos. 941,308 and 039,021.
Further, a focus detecting device provided with a plurality of distance measuring fields, and a focus detecting device having a wide range of field and an auxiliary light emission device suitable therefor are proposed in Japanese Laid-Open Patent Applications Nos. 63-78133, 63-78134, 63-82403, 63-82407 and 63-91645. By using a device having a plurality of distance measuring fields, focusing of a photo-taking lens on a main object lying at a desired position in the picture plane can be accomplished independently of the framing of a camera, and this leads to the advantage that the degree of freedom of the composition by an apparatus such as a still camera or a video camera is improved.
For example, in another application of the applicant's, a focus detecting device having three distance measuring points is constructed as shown in FIG. 5B of the accompanying drawings. FIG. 5A of the accompanying drawings depicts the whole of a single-lens reflex camera, and what is designated by A in FIG. 5A corresponds to the unit of FIG. 5B. The letter B denotes an auxiliary light emission unit. M1 designates a main mirror, M2 denotes a sub-mirror, P designates a pentaprism, E denotes an eyepiece, and F designates a film or a solid state image pickup element. The reference numeral 10 in the unit A denotes a field mask having rectangular openings 10a, 10b and 10c. These openings 10a, 10b and 10c determine parallel distance measuring fields, respectively. It is to be understood that the field mask 10 is disposed on or near the predetermined imaging plane of a zoom objective lens O in FIG. 5A.
The reference numeral 11 in FIG. 5B designates a two-aperture stop plate having openings 11a and 11b and having the function of dividing the exit pupil of the objective lens. The light fluxes of the same portion of an object passing through areas determined by these openings being reversely projected onto the exit pupil enter a photoelectric detector which will be described later.
The reference numeral 12 denotes a set of secondary imaging lenses which has positive lenses 12a and 12b. The set of secondary imaging lenses separates, for example, the portion of the object image restricted by the opening 10a vertically as viewed in FIG. 5B and re-images it. The object images formed by the secondary imaging lenses have the spacing therebetween changed in conformity with the focus-adjusted state of the objective lens. The two-aperture stop plate 11 and the set of secondary imaging lenses 12 are disposed in proximity to each other, but the design can also be made such that the marginal edge of the secondary imaging lenses serves also as a stop plate. It is desirable that a field lens be disposed near the field mask 10 and the two-aperture stop plate 11 be substantially imaged on the exit pupil of the objective lens. The reference numeral 13 designates a photoelectric detector provided with pairs of sensor arrays 13a and 13b, 13c and 13d, and 13e and 13f in the direction of arrangement of the secondary imaging lenses 12a and 12b. The direction of arrangement of the pairs of sensor arrays 13a and 13b, 13c and 13d, and 13e and 13f is orthogonal to the direction of division of the exit pupil of the objective lens. Instead of the sensor arrays being disposed so as to form pairs, the two ranges of one sensor array may be allotted. Each sensor array receives a light distribution based on the object image, and the photoelectric detector 13 outputs a signal conforming thereto.
The number of distance measuring fields with respect to the one set of secondary imaging lenses need not necessarily be three, but is determined by the imaging magnifications and the limits of the off-axis imaging performances of the imaging lenses 12a and 12b and how densely the sensor arrays can be disposed in a direction orthogonal to the direction of arrangement.
Besides what has been described above, a focus detecting device which functions equivalently to the device of FIG. 5B can also be constructed by juxtaposing a plurality of focus detecting optical systems each having one distance measuring field heretofore used, and such a technique is known.
On the other hand, an auxiliary light emission device for projecting a pattern onto a wide range of object surface, correspondingly to a focus detecting device having a plurality of distance measuring fields, has already been proposed. For example, the device of FIG. 6 of the accompanying drawings is such that a light emitted from a light emitting element 121 such as an LED illuminates a pattern chart 122 and the transmittance distribution shape of the pattern chart 122 is projected onto the object surface by a light projection lens 123. The LED element 121 has a convex spherical condensor lens 125 on the front face thereof and is adapted to efficiently direct the light from the light emitting chip 124 of the LED to the pattern chart. The pattern chart 122 has a stripe pattern extended in the lateral direction, i.e., covering all the distance measuring fields by a predetermined focal length, and can pattern-illuminate different object positions to which the plurality of distance measuring fields correspond, as shown in FIG. 7 of the accompanying drawings. In FIG. 7, A-D schematically show cases where the focal length of the photo-taking lens has been varied by zooming or the interchange of the lens, and in A, the focal length of the projection lens is long, and in D, the focal length of the projection lens is shortest. In each figure, the stripes in the lateral direction indicate the light quantity distribution pattern of the light projected by the light projection device, and the rectangles are object areas the fields of respective distance measuring points detect effectively. The shorter the focal length of the projection lens, the wider in angle is the picture plane and therefore, the object area each field detects becomes wider and the interval between the different distance measuring detection areas becomes greater. In FIG. 7, up to the state C are the limits in which the projected light pattern is effective for all of the three distance measuring points, and in the state D of the shorter focal length, only the middle distance measuring point is within the effective area of the projected light.
However, it has been found that the auxiliary light emission device of this type previously proposed suffers from the following problems. Firstly, to cope with the various focal length states of the photo-taking lens, it is necessary to project a pattern light onto as wide a range of the object field as possible, while on the other hand, the light energy projected onto a unit area is decreased and therefore, the effective reach distance of auxiliary light is decreased. The light power of a light source such as an LED which can be equipped in a portable camera apparatus is limited and therefore, this problem becomes more serious as the distance measuring field is enlarged to a wider range of the picture plane.
Secondly, with the usual photographic projection, it is often the case that the main object lies at the center of the picture plane, and when the frequency of use of each distance measuring point is taken into account, it is a great loss in use to project a light onto all the distance measuring points with an equal intensity of light. That is, it is desirable to distribute the light energy in accordance with the frequency of use of each distance measuring point.